In a conventional optical holographic technology, a portion of a coherent beam is used to irradiate to an object, and the reflected light from the object or the light transmitted through the object is incident to a holographic plate. This light is called as an object beam. Another portion of the coherent beam is projected onto the holographic plate to be used as a reference beam. Since the object beam and the reference beam are from the same coherent beam, the object beam and the reference beam are interfered with each other at the holographic plate to result in many bright and dark interference fringes. The interference pattern recorded on the holographic plate is formed as a hologram. The shape of the interference pattern indicates the phase relationship between the object beam and the reference beam. The bright/dark contrast level of the interference pattern indicates the intensity relationship between the object beam and the reference beam.
Conventionally, a disk-type multiplex holographic system uses a cylindrical lens to generate holograms. Due to the cylindrical lens, the hologram obtained at each shot is a long thin fan-shaped segment. Since the reconstructed image viewed by the observer is the combination of the images that are reconstructed from many small individual segments, the reconstructed image is usually suffered from distortion. Moreover, numerous vertical dark lines are superimposed on the reconstructed image, which is referred as a picket-fence effect.
FIG. 1 schematically illustrates the image exposure regions of the film obtained by an exposure procedure of a conventional disk-type multiplex holographic system. The conventional disk-type multiplex holographic system has a cylindrical lens (not shown). The cylindrical lens and a lens set (not shown) are collaboratively defined as an object beam imaging system. The film 200 is placed right on a focal plane of the object beam imaging system. When the object light wave corresponding to a first image of a target object and the reference light wave overlap on the film 200, these two light waves are interfered with each other to result in a long thin first image exposure region 2001. Similarly, when the object light wave corresponding to a second image of the target object and the reference light wave overlap on the film 200, these two light waves are interfered with each other to result in a long thin second image exposure region 2002. Since the film 200 is placed right on a focal plane of the object beam imaging system, a gap is easily formed between the first image exposure region 2001 and the second image exposure region 2002 if the offset movement of the film 200 is not matched with the width of the exposure region. Under this circumstance, the picket-fence effect occurs.
Moreover, in comparison with the conventional disk-type multiplex holographic system, a conventional image-plane disk-type multiplex holographic system utilizes a simplified optical system because the cylindrical lens is omitted. FIG. 2 schematically illustrates the image exposure regions of the film obtained by an exposure procedure of a conventional image-plane disk-type multiplex holographic system. By multiple exposures, the object image is imaged on a holographic film. As shown in FIG. 2, the film 220 has a first image exposure region 2201 and a second image exposure region 2202, wherein there is a large-area overlapped region 2203 between the first image exposure region 2201 and a second image exposure region 2202. The overlapped region 2203 has been subject to multiple exposures. As the subsequent image is recorded on the film 200, the overlapped region 2203 is continuously subject to many times of exposures. However, due to the multiple exposures of the film 220, the brightness of the reconstructed image is possibly insufficient. Under this circumstance, the holographic image is not aesthetically pleasing, and it is difficult to produce a large-sized reconstructed image.
Therefore, there is a need of providing an approach to solve the problems resulted from the multiple exposures of the film.